This invention relates to fiber-optic communications systems, and more particularly to prescription on the range of values of fiber dispersion for the design of systems operating with high-speed, high-power transmission.
With the advent of optical amplifiers which can compensate for fiber loss, the reach of fiber-optic communication systems at 10 Gb/s per channel and beyond, is limited largely by chromatic dispersion. Chromatic dispersion causes different parts of the signal spectrum to arrive at the distant end of the system at different times. An optical signal carrying information has a finite bandwidth (spread in wavelengths). If these wavelengths propagate at different velocities along the fiber, the pulses will be dispersed. The dominant cause of chromatic dispersion is material dispersion, the variation in the refractive index versus wavelength of silica, the basic material from which all low loss transmission fibers are made. However, the chromatic dispersion of a fiber can be tailored using waveguide dispersion. The magnitude of waveguide dispersion can be made as large or larger than the material dispersion.
Dispersion is especially deleterious in wavelength-division multiplexed (WDM) systems because the optical bandwidth required to accommodate multiple signals is wide relative to single-channel systems. Systems designed for C-band/L-band require transmission capability from 1530 to 1600 nm. Even wider band systems are now being contemplated and are likely to become commercial in the future.
To overcome the problem of dispersion of the signal before the advent of dispersion compensation a low-dispersion fiber called dispersion shifted fiber (DSF) has been invented. However, wavelength-division multiplexed (WDM) transmission suffers from non-linear distortion due to four-wave mixing (FWM) in DSFs. A fiber with moderate dispersion, referred to as non-zero dispersion shifted fiber (NZDSF) has been invented to reduce the effect of FWM. Such fiber, along with standard unshifted fiber (STD), requires dispersion compensation for proper transmission at 10 Gb/s per channel and above. However, NZDSF requires less dispersion compensation than STD fiber because NZDSF has a value of dispersion 3 to 4 times smaller than STD fiber (dispersion is xcx9c4 ps/(nm-km) at 1550 nm as opposed to xcx9c17 ps/(nm-km) for STD fiber).
For high-speed systems (40 Gb/s per channel and above, or 10 Gb/s with short pulses, i.e. 15 ps pulse duration or less) the effect of high dispersion of the transmission fiber is to broaden the pulses considerably so that neighboring pulses from the same channel overlap within each span. Conventional wisdom would suggest that one should avoid such pulse overlap as it will produce signal distortion due to non-linearity. However, it was found that it is possible to transmit a signal even in the presence of pulse overlap. This regime is referred to as pseudo-linear transmission. Even though pseudo-linear transmission allows pulse overlap during transmission, distortions from pulse-to-pulse nonlinear interaction is the limiting factor for transmission. Pulse distortion is the expected result from dispersion in the fiber for high-speed systems.
We have discovered that inherent fiber dispersion affects signals transmitted in the pseudo-linear regime in a manner completely different than signals transmitted by conventional techniques. Surprisingly, using the pseudo-linear regime, optical pulses actually transmit with lower distortion in fibers with higher dispersion values. Use of fibers with dispersion values of more than 20 ps/(nm-km) would not be considered suitable for high-speed systems based on conventional design principles. However, we have found for instance that fiber with a dispersion value higher than 20 ps/(nm-km), yields improved system performance in terms of reduced eye closure penalty at high signal power levels. Optical pulse transmission in the regime of interest, and the regime to which this invention is applicable, is termed pseudo-linear mode transmission (PLMT), which is becoming a known and accepted mode in the art. For the purpose of this description it is defined by, inter alia, a transmission bit rate of at least 10 Gb/s, using optical pulses at wavelengths of 1.25 to 1.65 xcexcm, with a duty cycle (pulse duration/pulse separation) of 10% to 50%, and a pulse duration of less than 15 ps.